Publications by authors named "Keren Shemtov-Yona"

9 Publications

  • Page 1 of 1

Quantitative assessment of the jawbone quality classification: A meta-analysis study.

PLoS One 2021 16;16(6):e0253283. Epub 2021 Jun 16.

Faculty of Mechanical Engineering, Technion, Haifa, Israel.

Aim: Bone quality is evaluated using bone density for qualitative classification, a characteristic that may be delicate to evaluate. Contemporary implantology that relies on modern measurement techniques, needs a more quantitative estimate of the bone quality.

Materials And Methods: PubMed and EMBASE databases were searched with no time restriction. Clinical and radiographic studies reporting on alveolar ridge dimensions and its parameters in different areas of the dentate and edentulous jaws were included. A meta-analysis was performed using random effect models to report a combined mean for alveolar ridge and its parameters. Meta regression statistical tests were performed in order to identify differences in those outcome parameters.

Results: 30 studies were included. The majority of the selected studies (total of 27) used live human subjects and CBCT to analyze alveolar ridge dimensions and its parameters. Using the combined mean obtained from the meta-analysis, a typical portrait of the alveolar ridge was constructed, and a geometrically based quantitative bone classification proposed. The quantitative classification was found to match the existing qualitative classification.

Conclusion: A geometry-based analysis was constructed that yields valuable insights on the bone type based on its components and on the dynamics of the dentate / edentulous states.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0253283PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8208540PMC
June 2021

A stochastic micro to macro mechanical model for the evolution of bone-implant interface stiffness.

Acta Biomater 2021 Jun 12. Epub 2021 Jun 12.

Department of Biomaterials, Sahlgrenska Academy, University of Gothenburg, Göteborg, Sweden.

Upon placement of an implant into living bone, an interface is formed through which various biochemical, biological, physical, and mechanical interactions take place. This interface evolves over time as the mechanical properties of peri-implant bone increase. Owing to the multifactorial nature of interfacial processes, it is challenging to devise a comprehensive model for predicting the mechanical behavior of the bone-implant interface. We propose a simple spatio-temporally evolving mechanical model - from an elementary unit cell comprising randomly oriented mineralized collagen fibrils having randomly assigned stiffness all the way up to a macroscopic bone-implant interface in a gap healing scenario. Each unit cell has an assigned Young's modulus value between 1.62 GPa and 25.73 GPa corresponding to minimum (i.e., 0) and maximum (i.e., 0.4) limits of mineral volume fraction, respectively, in the overlap region of the mineralized collagen fibril. Gap closure and subsequent stiffening are modeled to reflect the two main directions of peri-implant bone formation, i.e., contact osteogenesis and distance osteogenesis. The linear elastic stochastic finite element model reveals highly nonlinear temporal evolution of bone-implant interface stiffness, strongly dictated by the specific kinetics of contact osteogenesis and distance osteogenesis. The bone-implant interface possesses a small stiffness until gap closure, which subsequently evolves into a much higher stiffness, and this transition is reminiscent of a percolation transition whose threshold corresponds to gap closure. The model presented here, albeit preliminary, can be incorporated into future calculations of the bone-implant system where the interface is well-defined mechanically. STATEMENT OF SIGNIFICANCE: A simple, physically informed model for the mechanical characteristics of the bone-implant interface is still missing. Here, we start by extending the reported mechanical characteristics of a one cubic micrometre unit cell to a 250 µm long interface made of 1 µm thick layers. The stiffness of each cell (based on mineral content) is assigned randomly to mimic bone micro-heterogeneity. The numerical study of this interface representative structure allows for the simultaneous determination of the spatio-temporal evolution of the mechanical response at local (discrete element) and global (overall model) scales. The proposed model is the first of this kind that can easily be incorporated into realistic future models of bone-implant interaction with emphasis on implant stability and different loading conditions.
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http://dx.doi.org/10.1016/j.actbio.2021.06.019DOI Listing
June 2021

Modeling the effect of osseointegration on dental implant pullout and torque removal tests.

Clin Implant Dent Relat Res 2018 Oct 27;20(5):683-691. Epub 2018 Jul 27.

Technion, Faculty of Mechanical Engineering, 32000, Haifa, Israel.

Background: Osseointegration of dental implants is a key factor for their success. It can be assessed either by destructive (eg, pullout or torque extraction), or nondestructive methods (eg, resonant frequency analysis). However, as of today there is a scarcity of models that can relate the outcome of destructive tests to the level of osseointegration.

Purpose: To study various percentages of bone to implant bonding (tie) using finite element simulations. While evolutions of the bone mechanical properties are not explicitly taken into account, emphasis is put on the 3-dimensional variable extent of the bone-implant bonding, its statistical distribution, and its influence on the measurable extraction and torque loads, seeking to obtain a quantitative relationship.

Materials And Methods: We performed numerical simulations of randomly tied implants and calculated the evolution of the pullout force as well as that of the extraction torque.

Conclusion: Within simplifying assumptions for the osseointegration represented by a tie (as opposed to frictional) constraint, the results of this work indicate that the torque test is more discriminant than the extraction one, while both cannot really discriminate osseointegration levels below a relative variation of 20%.
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http://dx.doi.org/10.1111/cid.12645DOI Listing
October 2018

On stress/strain shielding and the material stiffness paradigm for dental implants.

Clin Implant Dent Relat Res 2017 Oct 13;19(5):935-943. Epub 2017 Jun 13.

Faculty of Mechanical Engineering, Technion, Haifa, 32000, Israel.

Background: Stress shielding considerations suggest that the dental implant material's compliance should be matched to that of the host bone. However, this belief has not been confirmed from a general perspective, either clinically or numerically.

Purpose: To characterize the influence of the implant stiffness on its functionality using the failure envelope concept that examines all possible combinations of mechanical load and application angle for selected stress, strain and displacement-based bone failure criteria. Those criteria represent bone yielding, remodeling, and implant primary stability, respectively MATERIALS AND METHODS: We performed numerical simulations to generate failure envelopes for all possible loading configurations of dental implants, with stiffness ranging from very low (polymer) to extremely high, through that of bone, titanium, and ceramics.

Results: Irrespective of the failure criterion, stiffer implants allow for improved implant functionality. The latter reduces with increasing compliance, while the trabecular bone experiences higher strains, albeit of an overall small level. Micromotions remain quite small irrespective of the implant's stiffness.

Conclusion: The current paradigm favoring reduced implant material's stiffness out of concern for stress or strain shielding, or even excessive micromotions, is not supported by the present calculations, that point exactly to the opposite.
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http://dx.doi.org/10.1111/cid.12509DOI Listing
October 2017

Fatigue of Dental Implants: Facts and Fallacies.

Dent J (Basel) 2016 May 24;4(2). Epub 2016 May 24.

Faculty of Mechanical Engng, Technion, Haifa 32000, Israel.

Dental implants experience rare yet problematic mechanical failures such as fracture that are caused, most often, by (time-dependent) metal fatigue. This paper surveys basic evidence about fatigue failure, its identification and the implant's fatigue performance during service. We first discuss the concept of dental implant fatigue, starting with a review of basic concepts related to this failure mechanism. The identification of fatigue failures using scanning electron microscopy follows, to show that this stage is fairly well defined. We reiterate that fatigue failure is related to the implant design and its surface condition, together with the widely varying service conditions. The latter are shown to vary to an extent that precludes devising average or representative conditions. The statistical nature of the fatigue test results is emphasized throughout the survey to illustrate the complexity in evaluating the fatigue behavior of dental implants from a design perspective. Today's fatigue testing of dental implants is limited to ISO 14801 standard requirements, which ensures certification but does not provide any insight for design purposes due to its limited requirements. We introduce and discuss the random spectrum loading procedure as an alternative to evaluate the implant's performance under more realistic conditions. The concept is illustrated by random fatigue testing in 0.9% saline solution.
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http://dx.doi.org/10.3390/dj4020016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5851262PMC
May 2016

An Overview of the Mechanical Integrity of Dental Implants.

Biomed Res Int 2015 25;2015:547384. Epub 2015 Oct 25.

Faculty of Mechanical Engineering, Technion, Israel Institute of Technology, 32000 Haifa, Israel.

With the growing use of dental implants, the incidence of implants' failures grows. Late treatment complications, after reaching full osseointegration and functionality, include mechanical failures, such as fracture of the implant and its components. Those complications are deemed severe in dentistry, albeit being usually considered as rare, and therefore seldom addressed in the clinical literature. The introduction of dental implants into clinical practice fostered a wealth of research on their biological aspects. By contrast, mechanical strength and reliability issues were seldom investigated in the open literature, so that most of the information to date remains essentially with the manufacturers. Over the years, implants have gone through major changes regarding the material, the design, and the surface characteristics aimed at improving osseointegration. Did those changes improve the implants' mechanical performance? This review article surveys the state-of-the-art literature about implants' mechanical reliability, identifying the known causes for fracture, while outlining the current knowledge-gaps. Recent results on various aspects of the mechanical integrity and failure of implants are presented and discussed next. The paper ends by a general discussion and suggestions for future research, outlining the importance of mechanical considerations for the improvement of their future performance.
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http://dx.doi.org/10.1155/2015/547384DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4637045PMC
September 2016

The effect of oral-like environment on dental implants' fatigue performance.

Clin Oral Implants Res 2014 Feb 20;25(2):e166-70. Epub 2012 Dec 20.

Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel.

Aim And Objectives: The aim of this study was to evaluate the influence of fluid environment mimicking intra-oral conditions on fatigue performance of standard diameter, 3.75-mm implants. Dental implants placed intra-orally are repeatedly submitted to mastication loads in the oral environment, which differ substantially from room-air standard laboratory conditions. Several studies that examined fracture surfaces of intra-orally fractured dental implants have identified corrosion fatigue as the main failure mechanism. Yet, fatigue performance of dental implants has been essentially studied in room air, based on the premise that the implant material is relatively resistant to corrosion in the intra-oral environment.

Material And Methods: Thirty-two 3.75-mm titanium alloy implants were tested under cyclic compressive loading. The tests were performed in artificial saliva substitute containing 250 ppm of fluoride. The loading machine stopped running when the implant structure collapsed or when it completed 5 × 10(6) cycles without apparent failure. The load vs. number of cycles was plotted as curve for biomechanical fatigue analysis (S-N curve). The S-N curve plotted for the artificial saliva test was compared to the curve obtained previously for the same implants tested in a room-air environment. Failure analysis was performed using scanning electron microscopy (SEM).

Results: A comparison of the S-N curves obtained in artificial saliva and in room air showed a considerable difference. The S-N curve obtained in the artificial saliva environment showed a finite life region between 535N and 800N. The transition region was found below 465N, with a probability of survival of 50%, while in room air, the transition region was between 810N and 620N and an infinite life region below 620N was identified.

Conclusions: The results of this study show that environmental conditions adversely affect implants' fatigue performance. This fact should be taken into account when evaluating the mechanical properties of dental implants.
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http://dx.doi.org/10.1111/clr.12084DOI Listing
February 2014

Effect of dental implant diameter on fatigue performance. Part I: mechanical behavior.

Clin Implant Dent Relat Res 2014 Apr 10;16(2):172-7. Epub 2012 Jul 10.

Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel.

Aim: The purpose of this study was to evaluate the effect of the implants' diameter on the mechanical function and load-fatigue performance of dental implants.

Materials And Methods: Three groups of implants with different diameters (3.3 mm, 3.75 mm and 5 mm), were tested under static and cyclic compressive loading. A total number of 15 implants for the static test and 112 implants for the cyclic-fatigue test. In the cyclic test, the machine ceased operating when the structure collapsed or when it reached 5 × 10(6) cycles without apparent failure. The load versus the number of cycles was plotted as curves for biomechanical analysis (S-N curve) for each implant diameter.

Results: The S-N curve plotted for the 5 mm implants showed classic fatigue behavior with a finite life region starting from 620N. The same was observed for the 3.75 mm diameter implants, with a finite life region starting below 620N. By contrast, the 3.3 mm diameter implants failed to show predictable fatigue behavior and a fatigue limit could not be defined.

Conclusions: The results of this study emphasize the importance of implant diameter on fatigue behavior. Narrow implants failed to show typical fatigue behavior which might be attributed to the implant design.
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http://dx.doi.org/10.1111/j.1708-8208.2012.00477.xDOI Listing
April 2014

Effect of dental implant diameter on fatigue performance. Part II: failure analysis.

Clin Implant Dent Relat Res 2014 Apr 10;16(2):178-84. Epub 2012 Jul 10.

Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel.

Purpose: The purpose of this study was to perform fracture mode analysis for in vitro failed implants in order to evaluate the relation between the fracture mode obtained and the implants' fatigue behavior.

Materials And Methods: Eighty fractured dental implants were analyzed after being tested for fatigue performance. A macroscopic failure analysis was performed, which evaluated and located the fracture modes obtained, followed by a microscopic failure analysis comprising a detailed scanning electron microscopy (SEM) fractographic analysis.

Results: Four distinctive fracture loci were identified and macrofracture mode analysis was performed, showing that all 5-mm implants that fractured were fractured at the abutment neck and screw. In the 3.75-mm group, 44.4% were fractured at the implant neck and 55.5% at the implants second thread. Fifty-two percent of the 3.3-mm fractured implants had it at the implants second tread and 48% at the implants third thread. The implant's metallographic sections revealed that the different fracture loci were located where thin metal cross sections and sharp notches coexist. Using SEM, we were able to characterize the failure micromechanisms and fatigue characterization as transgranular fracture and arrays of secondary parallel microcracks at relatively low magnifications and classic fatigue striations at much higher magnifications.

Conclusions: The results of this study indicate that proper implant design is crucial to ensure long-term fatigue performance for dental implants. The combination of sharp notches (thread) and narrow metal cross section is quite deleterious for fatigue resistance.
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http://dx.doi.org/10.1111/j.1708-8208.2012.00476.xDOI Listing
April 2014
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